The present invention relates to semiconductor thin film processing by nanolayer deposition (NLD). The fabrication of modern semiconductor device structures has traditionally relied on plasma processing in a variety of operations such as etching and deposition. In plasma etching, for example, chemically reactive gas molecules and energetic ions are used to remove material from a substrate. Plasmas are also used in a number of deposition techniques such as Physical Vapor Deposition (PVD) and Plasma Enhanced Chemical Vapor Deposition (PECVD).
In PVD processes, also known as sputter deposition processes, plasmas are necessary to ionize gas molecules that are driven into a sputter target to generate a vapor of atoms and clusters of atoms that are subsequently deposited onto a substrate. PVD processes are typically performed in a high vacuum apparatus.
PVD is a line-of-sight deposition process in which material sputtered from the source target tends to preferentially adhere to surfaces that are exposed directly to the target with little or no reflection from surfaces inside the vacuum chamber or on the surface of the substrate. In line-of-sight deposition systems, such as PVD systems, the formation of conformal layers over topographical features with aspect ratios of greater than 2-4 to 1 is, therefore, difficult or not possible to achieve.
In CVD, a gas or vapor mixture consisting of one or more precursors is flowed over the surface of a substrate that is heated to an elevated temperature. Reactions then occur at the hot surface resulting in a film of deposited material. The temperature of the wafer surface is an important parameter in CVD that affects the film properties and deposition rates. High temperatures of 350° C. and higher can be required for thermal CVD processes and these temperatures may not be compatible with other processes in an integrated semiconductor fabrication process flow. As a result of this temperature limitation, plasmas have been introduced into CVD processes to enhance the deposition rate, and provide for improved film quality at lower substrate temperatures. More details on PVD and CVD methods are discussed in International Publication Number WO 00/79019 A1 or PCT/US00/17202 to Gadgil, the content of which is incorporated by reference.
An alternative to PVD and CVD is atomic layer deposition (ALD). ALD is a cyclic deposition process in which a first reactive precursor gas is injected into a chamber, for durations as short as 100-500 milliseconds, to adsorb onto a substrate and in which a second reactive precursor gas is then injected to react with the first precursor gas on the substrate to form a thin layer of deposited film. The ALD process consists of alternating steps of evacuating a chamber, introducing a first precursor to form a monolayer or less on a substrate, again evacuating the chamber, and introducing a second precursor to react with the first precursor on the substrate to form a thin layer of deposited film. This alternating sequence is repeated until a desired film thickness is achieved. In ALD, the substrate is generally heated to drive the reaction between the second and the first precursors and can be as high as 400° C. for some processes, or higher.
U.S. Pat. No. 5,916,365 to Sherman, entitled “Sequential chemical vapor deposition” provides for sequential chemical vapor deposition by employing a reactor operated at low pressure, a pump to remove excess reactants, and a line to introduce gas into the reactor through a valve. Sherman teaches exposing the part to be coated to a gaseous first reactant, including a non-semiconductor element of the thin film to be formed, wherein the first reactant adsorbs on the part to be coated. The deposited film thickness in each cycle is a monolayer or less. The first reactant forms a monolayer on the part to be coated (after multiple cycles), while the second reactant passes through a radical generator which partially decomposes or activates the second reactant into a gaseous radical before it impinges on the monolayer. This second reactant does not necessarily form a monolayer, but is available to react with the monolayer. A pump removes the excess second reactant and reaction products completing the process cycle. The process cycle can be repeated to grow the desired thickness of film.
U.S. Pat. No. 6,200,893 to Sneh entitled “Radical-assisted sequential CVD” discusses a method for CVD on a substrate wherein radical species are used in alternate steps to deposit a film from a molecular precursor, to treat the material deposited from the molecular precursor, and to prepare the substrate surface with a reactive chemical in preparation for the subsequent molecular precursor step. By repetitive cycling, a composite integrated film is produced. In a preferred embodiment, the deposited layers from the molecular precursor are metals, and the radicals in the alternating treatment steps are used to remove ligands remaining from the metal precursor reactions, and to oxidize or form a nitride of the metal surface in subsequent layers.
In one embodiment taught by Sneh, a metal is deposited onto a substrate surface in a deposition chamber by: (a) depositing a monolayer of metal on the substrate surface by flowing a molecular precursor gas or vapor bearing the metal over a surface of the substrate, the surface saturated by a first reactive species with which the precursor will react by depositing the metal and forming reaction products, leaving a metal surface covered with ligands from the metal precursor and therefore not further reactive with the precursor; (b) terminating flow of the precursor gas or vapor; (c) purging the precursor with inert gas; (d) flowing at least one radical species into the chamber and over the surface, the radical species highly reactive with the surface ligands of the metal precursor layer and eliminating the ligands as reaction product, and also saturating the surface, providing the first reactive species; and (e) repeating the steps in order until a metallic film of desired thickness is achieved.
In another aspect of the Sneh disclosure, a metal nitride is deposited on a substrate surface in a deposition chamber by: (a) depositing a monolayer of metal on the substrate surface by flowing a metal precursor gas or vapor bearing the metal over a surface of the substrate, the surface saturated by a first reactive species with which the precursor reacts by depositing the metal and forming reaction product, leaving a metal surface covered with ligands from the metal precursor and therefore not further reactive with the precursor; (b) terminating flow of the precursor gas or vapor; (c) purging the precursor with inert gas; (d) flowing a first radical species into the chamber and over the surface, the atomic species highly reactive with the surface ligands of the metal precursor layer and eliminating the ligands as reaction product and also saturating the surface; (e) flowing radical nitrogen into the chamber to combine with the metal monolayer deposited in step (a), forming a nitride of the metal; (f) flowing a third radical species into the chamber terminating the surface with the first reactive species in preparation for a next metal deposition step; and (g) repeating the steps in order until a composite film of desired thickness is achieved.
In the Sneh embodiments, monolayers or fractions of monolayers are deposited in each cycle of a multi-cycle process and, as a result, the deposition of typical film thicknesses on the order of hundreds of angstroms using the Sneh process can be slow.
As discussed in connection with the Sherman and Sneh patents, above, the ALD process includes cycles of flowing gas reactant into a chamber, adsorbing a layer of gas onto a wafer surface, purging the gas reactant, flowing a second gas reactant into the chamber, and reacting the second gas reactant with the first gas reactant to form a monolayer, or sub-monolayer on the wafer surface. Thick films are achieved by deposition of multiple cycles.
Precise film thicknesses in ALD can be controlled by controlling the number of cycles in the deposition process, since a controlled thickness of material is deposited in each step of the process.
The throughput in terms of the number of substrates that can be processed per unit time in device fabrication environments for conventional ALD systems can be less than 1 wafer per hour in configurations in which single wafers are processed at a time and in which a targeted film thickness is in the range of 10-50 nm. ALD process chambers can be designed with minimal volume to minimize cycle times and maximize throughput, although a reduction in the process volume will not affect the number of cycles required to achieve typical film thicknesses. The characteristic self-limiting surface adsorption reaction of ALD results in a process that is considerably slower than CVD.